Add logger interface and stop relying on Logrus directly

cmd/dex/config.go

cmd/dex/serve.go

connector/authproxy/authproxy.go

connector/bitbucketcloud/bitbucketcloud.go

connector/github/github.go

connector/gitlab/gitlab.go

connector/keystone/keystone.go

connector/ldap/ldap.go

connector/ldap/ldap_test.go

connector/linkedin/linkedin.go

connector/microsoft/microsoft.go

connector/mock/connectortest.go

connector/oidc/oidc.go

connector/saml/saml.go

connector/saml/saml_test.go

pkg/log/logger.go

pkg/log/logrus.go

pkg/log/logrus_test.go

server/api.go

server/api_test.go